1. Field of the Invention
The present invention relates to a tomographic image processing method, apparatus and program for producing tomographic images (optical tomographic images) by OCT (Optical Coherence Tomography) measurement and a system for producing optical tomographic images using the method, the apparatus or the program.
2. Description of the Related Art
Conventionally, an optical tomographic image obtainment apparatus utilizing OCT measurement has been used in some cases to obtain tomographic images of tissue in vivo (in living organisms) by optical tomography. The optical tomographic image obtainment apparatus is mainly used to observe the eyegrounds (fundi of eyeballs or ocular fundi), anterior segments of eyeballs and skin of patients. Further, the optical tomographic image obtainment apparatus is used to observe various regions of patients. For example, the optical tomographic image obtainment apparatus is used to observe the walls of arteries using a fiber probe and to observe digestive organs by inserting a fiber probe through a forceps channel of an endoscope. In the optical tomographic image obtainment apparatus, low coherent light that has been output from a light source is divided into measurement light and reference light. Then, a measurement target is irradiated with the measurement light, and reflection light that is reflected from the measurement target or backscattered light is combined with the reference light. Further, an optical tomographic image is obtained based on the intensity of interference light between the reflection light and the reference light.
The OCT measurement can be classified into two types, namely, TD-OCT (Time Domain OCT) measurement and FD-OCT (Fourier Domain OCT) measurement. In the TD-OCT (Time Domain OCT) measurement disclosed in Japanese Unexamined Patent Publication No. 2001-264246, the intensity of interference light is measured while the optical path length of reference light is changed. Accordingly, the distribution of the intensities of reflection light that corresponds to the depth-direction position of a measurement target (hereinafter, referred to as a depth position) is obtained.
Meanwhile, in the FD-OCT (Fourier Domain OCT) measurement, the optical path length of reference light and that of signal light are not changed, and the intensity of interference light is measured for each spectral component of light. Further, a frequency analysis, such as Fourier transformation, is performed on the obtained spectral interference intensity signal by using a computer. Accordingly, the distribution of the intensities of reflection light that corresponds to the depth position is obtained. Since the FD-OCT measurement does not require mechanical scan that is necessary in the TD-OCT measurement, the FD-OCT measurement enables high-speed measurement. Therefore, in recent years, the FD-OCT measurement has drawn considerable attention.
Typical examples of the apparatus for performing FD-OCT (Fourier Domain OCT) measurement are an SD-OCT (Spectral Domain OCT) apparatus and an SS-OCT (Swept Source OCT) apparatus. In the SS-OCT apparatus, laser light, the wavelength of which is temporally swept, is output from a light source unit, and reflection light and reference light are interfered with each other at each wavelength. Further, the temporal waveform of signals corresponding to temporal changes in the frequency of light is measured. Accordingly, spectral interference intensity signals are obtained. Further, Fourier transformation is performed on the spectral interference intensity signals by using a computer to obtain an optical tomographic image (please refer to Japanese Unexamined Patent Publication No. 2006-132996).
In the SS-OCT measurement, a method for improving the S/N ratio is disclosed. In the method, an optical fiber coupler or the like is used, and interference light is split into two so that the light amounts of the split interference light are substantially equal to each other. Further, each of the split interference light is detected by a detector, and a difference between detected signals is detected as an interference signal (balanced detection), as disclosed in Japanese Unexamined Patent Publication No. 2001-264246. According to the method, the interference signal is amplified to twice the value thereof, and same-phase optical noise, other than the interference signals, is cancelled. Hence, the non-interference components are removed, and the S/N ratio is improved.
In the aforementioned TD-OCT measurement disclosed in Japanese Unexamined Patent Publication No. 2001-264246, so-called balanced detection is performed on the interference light to improve the S/N ratio. Further, the balanced detection may be applied to the SS-OCT measurement disclosed in Japanese Unexamined Patent Publication No. 2006-132996. To improve the S/N ratio by the balanced detection, when the interference light is split into two, the interference light must be split in such a manner that the light amounts of the interference light after splitting are substantially equal to each other. Further, it is necessary that the optical path length of one of the split interference light and that of the other split interference light are the same.
However, there are cases in which the two optical path lengths (phase difference) differ from each other. In that case, there is a problem that the S/N ratio drops.